Apparatus for use in the coating of electrically conducting members by the process ofelectrophoresis



T. JACKSON ETAL 3,094,477

THE COATING OF ELECTRICALLY ERS BY THE PROCESS OF E TROPHORESIS Fi Oct. 9, 1961 June 18, 1963 APPARATUS FOR USE IN CONDUCTING MEMB United rates Patent Office 3,@ -i,i77. Patented June 18, 1963 3 47 7 APPARATUS FOR USETN THE COATlNG OF ELEC- TRICALLY CONDUCTING MEMBERS BY THE PROCESS OF ELECTRQPHORESIS Thomas Jackson, South Beats, Sunderland, and Alexander This invention relates to apparatus for coating, by the process of electrophoresis, a selected portion, or portions, of one or more electrically conducting members with one or more suitable materials.

The invention is particularly, but not exclusively, applicable to the coating of tubular conducting members, employed in the construction of indirectly heated cathodes, with a material capable of being rendered thermionically emissive.

An indirectly heated cathode for a thermionic device may comprise a tubular member, coated externally with thermionically emissive material, with a heater, in the form of a filament, located within the member. For convenience in manufacture of the thermionic device supplying the cathode, and to enable terminal wires to be connected electrically to an end portion of the tubular memher, it is usually necessary to restrict the thermionically emissive material to the central region of the member.

In a known method of applying thermionically emissive material to a tubular member, a plurality of members are supported in a jig which also masks their end portions, and the material which is capable of being rendered thermionically emissive, is mixed with a suitable liquid to form a slurry which is sprayed onto the members, but which is prevented from coating the masked portions thereof. To ensure uniform and predictable operation of the cathodes, it is necessary to apply the material evenly over the whole unmasked surface of the member to form a layer having the required thickness. It is particularly difficult to spray the material on to the adjacent portions of members located side-by-side in the jig, and unless the mask is close contact with the entire periphery of the member, a ragged edge is formed between the sprayed and nonsprayed portions of the members.

The application of a coating to an electrically conducting member by the process of electrophoresis is known. In this process, the member to be coated is immersed in a suspension containing the coating material and a substance which imparts to the particles of the coating material an electric charge, such that when a potential of the required polarity is applied between the member to be coated and an electrode dipped into the suspension, the particles of the coating material are deposited on the member in the form of a uniform layer. Any part of the member on which the coating is not required has to be masked, and if the member is hollow, care has to be taken to prevent a coating from being formed on the inside surface of the member.

An object of the present invention is to provide apparatus for simultaneously applying a coating to a plurality of electrically conducting members by the process of electrophoresis, and which overcomes the above-mentioned difficulties.

According to one aspect of the present invention, apparatus for use in the process of coating by electrophoresis a selected portion or portions of at least one electrically conducting member with at least one finely divided material from a liquid suspension thereof, comprises a rotatable cylindrical support, a plurality of rows of raised projections, formed from electrically insulating material,

and between which the electrically conducting member(s) are supportable, said projections being spaced peripherally on said support or on a flexible band in contact with the periphery thereof, and arranged in parallel relation in the direction of movement of the support, the spacing apart of the rows of projections defining the portion(s) of the member(s) to be coated, and a trough formed of electrically insulating material into which a segmental portion of the support extends, said trough having a plurality of ribs which co-act with said rows of projections to define compartments in said trough, alternate compartments being adapted to contain a suspension of a coating material to be applied to said conducting member(s) and provided with an electrode which dips into said suspension, the remaining compartments being adapted to contain an electrically conducted liquid through which electrical contact with said member(s) is established and which assists in confining the suspensions to their respective compart ments.

According to a further aspect of the invention, apparatus for use in the process of coating by electrophoresis a selected portion of at least one electrically conducting member with a finely divided material from a liquid suspension thereof, comprises a rotatable cylindrical support, two rows of raised projections, formed from electrically insulating material, and between which the electrically conducting member(s) are supportable, said projections being spaced peripherally on said support and arranged in parallel relation in the direction of rotation of the support, the spacing apart of the rows of projections defining the portion of the member( s) to be coated, and a trough formed of electrically insulating material into which a segmental portion of the support extends, said trough having ribs which co-act with the rows of circumferentially raised projections on said support to define three compartments in said trough, the intermediate compartment being adapted to contain a suspension of the coating material to be applied to the conducting member(s) and having an electrode dipped therein, the outer compartments of said trough being adapted to contain an electrically conducting liquid through which electrical contact with said member(s) is established and which assists in confining the suspension to said intermediate compartment.

The rows of raised projections may be supported on a cylindrical drum which is rotatable about its axis, such that a segmental portion of the drum dips into the associated trough. Alternatively, the projections may be supported on a flexible band of insulating material which passes around the perimeter of the rotatable cylindrical support and a further cylindrical support which is spaced apart therefrom.

The coating is conveniently applied to the conducting member by the process of cataphoresis and in this process a positive electrical potential is applied to electrodes dipped into the suspension(s) of coating material and a negative potential is applied to the conducting liquid. As the support is moved, or rotated, the members are in turn introduced into, and removed from, the liquids contained in the compartments. When a portion of an electrically conducting member is in contact with the conducting liquid the member takes up the potential of that liquid and the portion(s) of the member in contact with the suspension liquids have a coating formed thereon by the process of cataphoresis. The surface tension effect of the electrically conducting liquid, which is preferably mercury, prevents the suspension liquids from spreading between the rows of raised projections and the ribs forming the compartments.

In one embodiment of the invention the cylindrical support is in the form of a drum, and the trough is provided with two ribs which divide it into three compartments.

The outer compartments are filled with mercury and the coating material is in suspension in the intermediate compartment. As the drum is rotated a layer of material is deposited on the centre portion of the member(s) as they pass through the trough.

. In a further embodiment of the invention, the trough is divided into five separate compartments, the two outer compartments contain a cataphoretic coating suspension of one type, the inner compartment contains a suspension of a different type, and the intermediate compartments contain mercury. This arrangement enables two different materials to be simultaneously deposited on different parts of an electrically conducting member immersed in said compartments.

The support is conveniently in the form of a cylindrical drum formed from an electrically insulating material which is unafiected by the materials contained in the coating suspensions. Nylon has been found to be completely satisfactory.

In order that the invention may be more readily understood, it will now be described by way of example with reference to the accompanying drawings, in which:

FIG. 1 shows diagrammatically apparatus according to one embodiment of the present invention,

FIG. 2 is an enlarged fragmentary view of the raised projections shown in FIG. 1,

FIG. 3 shows diagrammatically apparatus according to a further embodiment of the invention,

FIG. 4 shows diagrammatically apparatus according to a still further embodiment of the invention, and

FIG. 4A is a detailed perspective view of the still further embodiment of the invention.

FIG. 1 shows diagrammatically apparatus for applying a coating of material to a central portion of an electrically conducting member, such as a tubular sheath for an indirectly heated cathode, by the process of cataphoresis. Reference numeral 1 indicates a cylindrical support conveniently formed from an electrical-1y insulating material and consisting of a cylindrical drum which is rotatable about its axis such that a segmental portion of the support dips into a trough 2. The support is provided on its periphery with two rows of raised circumferential projections 3 which are formed electrically insulating material and between which conductor members 4 to be coated are supported. The spacing apart of the rows of raised projections defines the portion of the member to which the coating will be applied. A pair of discs 12 may be provided, one on each side of the drum, to help locate the members 4 between the raised projections.

The trough 2, which is formed from electrically insulating material, is provided with raised ribs 5 which co-act with the rows of raised circumferential projections 3 on the periphery of the support, to divide the trough into three separate compartments, into each of which a part of the conducting members extend when the portion of the support to which they are attached dips into the trough.

The coating material to be applied to the members may be material capable of conversion to a thermionically emissive material and it is made into a suitable cataphoretic suspension in a. manner known in the art and is placed in the intermediate compartment 6 of the trough. An electrode 7, to which a positive electrical potential may be applied, dips into the suspension contained in the intermediate compartment 6. The outer compartments 8 contain an electrically conducting liquid, conveniently mercury, which enables electrical contact to be made to the conducting members, and which, by virtue of its surface tension effect, prevents the suspension from leaking from the intermediate compartment 6 to the outer compartments 8. The level of the suspension in the intermediate compartment should be equal to, or slightly lower than, that of the mercury in the outer compartments, such that deposition of the material from the suspension occurs only by cataphoresis,

An electrode 9, to which a negative potential is applied, dips into the conducting liquid in either, or both, of the outer compartments, and as the drum 1 is rotated the conducting members mounted thereon in turn come into contact with the suspension and the conducting liquid and a coating of the material in suspension is deposited cataphoretically on the central region of the conducting members as they are immersed in the suspension. The conducting liquid blanks off the end portions of the members and prevents material from being deposited over the end portions.

The speed of rotation of the drum, and the current supplied to the electrodes determine the thickness of the coating which is formed on the members.

The raised projections 3 on the surface of the drum, and the co acting ribs 5, form the compartments of the trough, and also define the extent to which the coating material is applied to the members. In order to provide a sharp cut-ofi region between the coated and uncoated sections of the members, care has to be taken to prevent suspension contained in the intermediate compartment from leaking into the outer compartments. Leakage from the intermediate compartment is prevented by supporting the conducting members between the raised projections, and a portion of each projection is tapered on one side 10, and is flat on the other side 11, as shown in FIG. 2. The fiat sides 11 of the projections are arranged to be in contact with the conducting liquid and the surface tension of the liquid is such that the suspension is prevented from leaking away from the intermediate compartment. The ribs 5 are also tapered as shown in FIG. 2.

As stated above, the drum and the trough are formed from electrically insulating material, but care has to be taken to ensure that the insulating material is not chemically affected by the materials contained in the cataphoretic suspension. Nylon is a particularly suitable material.

FIG. 3 shows a further embodiment of the invention, in which the drum is provided with four substantially parallel rows of spaced pnojections 3. The trough is similarly provided with four co-acting ribs 5' and it is divided into five compartments. This arrangement enables a coating to be applied to three selected portions of the member and the coatings may be of different materials. For example, while the coating on the external surface of the tube is of material capable of conversion to a thermionically emissive material, that on the interior Surface of the tube is of insulating material. The coating materials are made up into cataphoretic suspensions and are placed in the two outer and the central compartments. The alternate compartments contain the electri-' cally conducting liquid. This liquid again prevents the suspensions from mixing and also enables a negative electrical potential to be applied to the conducting members. If the conducting members are in the form of hollow tubes, the suspension contained in the outer compartments may be made to flow through the tube and cataphoretically coat the inside of the tube, and the other coating is applied to the central outer region of the tube. In this case, space must be left between the ends of the tubular members and the discs 12' to allow the suspension contained in the outer compartments to flow through the tubes.

In a further embodiment of the invention a five compartment trough is obtained by forming two ribs and two rows of co-acting raised projections on the drum, each rib and projection is provided with raised edges and a hollow central portion. The conducting liquid is contained in the hollow portions of the ribs and the suspensions are contained between the ribs, and in the outer compartments.

In a still further embodiment of the invention, as illustrated in FIG. 4, an endless flexible band 13 of electrically insulating material, is supported by a cylindrical drum 14 and a further drum 15 spaced apart therefrom. The

band is provided on its outer surface with the required raised projections 16 which support the members and co-act with the ribs 17 formed in an associated trough 20 to form the necessary compartments. It is preferable that the axes of the drums are arranged in a substantially vertical plane, such that the portion of the band which is caused to pass through the trough as the lower drum is rotated is supported by part of the periphery of that drum.

What we claim is:

1. Apparatus for use in the process of coating at least one selected portion of at least one electrically conducting member with at least one finely divided material from a liquid suspension thereof by electrophoresis; which comprises a cylindrical support, a plurality of rows of raised, electrically insulated projections spaced peripherally on said support for supporting the conducting members to be coated, said rows of projections being in spaced parallel relation normal to the longitudinal axis of the support with the spacing therebetween defining the portion of the members to be coated, an electrically insulated trough positioned below said support for a distance such that a lower segmental portion of the support extends into the trough, a plurality of vertically disposed ribs positioned in said trough and coacting with said rows of projections to define a plurality of compartments in said trough with alternate compartments thereof being adapted to contain suspensions of coating material and the remaining compartments thereof being adapted to contain an electrically conducting liquid which serves to enable electrical contact to be made with said conducting members and assist in confining said suspensions of coating material to their respective compartments, and an electrode extending into each compartment.

2. Apparatus for use in the process of coating at least one selected portion of at least one electrically conducting member with at least one finely divided material from a liquid suspension thereof by electrophoresis; which comprises a first cylindrical support rotatable about its longitudinal axis, and a second cylindrical support located in spaced, parallel relationship therewith, a flexible band in contact with the peripheries of said support, a plurality of rows of raised, electrically insulated projections spaced peripherally on said band for supporting the conducting members to be conducted, said rows of projections being arranged in planes normal to the longitudinal axis of the support with the spacing therebetween defining the portion of the members to be coated, an electrically insulated trough positioned below said sup port for a distance such that a lower segmental portion of the support extends into the trough, a plurality of vertically disposed ribs positioned in said trough and coacting with said rows of projections to define a plurality of compartments in said trough with alternate compartments thereof being adapted to contain suspensions of coating material and the remaining compartments thereof being adapted to contain an electrically conducting liquid which serves to enable electrical contact to be made with said conducting members and assist in confining said suspensions of coating material to their respective compartments, and an electrode extending into each compartment.

3. Apparatus for use in the process of coating a selected portion of at least one electrically conducting member with a finely divided material from a liquid suspension thereof by electrophoresis, which comprises a cylindrical drum rotatable about its longitudinal axis, two rows of raised, electrically insulated projections spaced peripherally on said drum for supporting the conducting members to be coated, said rows of projections being in spaced, parallel relationship normal to the longitudinal axis of the drum with the spacing therebetween defining the portion of the members to be coated, an electrically insulated trough positioned below said drum for a distance such that a lower segmental portion of the drum extends into the trough, a plurality of vertically disposed ribs positioned in said trough and coacting with said rows of projections to define three compartments in said trough with the intermediate compartment being adapted to contain a suspension of the coating material and the two outer compartments being adapted to contain an electrically conducting liquid which serves to enable electrical contact to be made with said conducting member and assist in confining said suspension to the intermediate compartment, and an electrode extending into each of the three compartments.

4. Apparatus for use in the process of coating a selected portion of at least one electrically conducting member with a finely divided material from a liquid suspension thereof by electrophoresis, which comprises a first cylindrical drum rotatable about its longitudinal axis, a second rotatable cylindrical drum located in spaced, parallel relationship therewith, a flexible band in driving contact with the peripheries of said drums, two rows of raised, electrically insulated projections spaced peripherally on said drums for supporting the conducting members to be coated, said rows of projections being in spaced, parallel relationship normal to the longitudinal axis of said drums with the space therebetween defining the portion of the members to be coated, an electrically insulated trough positioned below said drums for a distance such that a lower segmental portion of the drum extends into the trough, a plurality of ribs positioned in said trough and coacting with said rows of projections to define three compartments in said trough with the intermediate compartrnent being adapted to contain a suspension of coating material and the two outer compartments being adapted to contain an electrically conducting liquid which serves to enable electrical contact to be made with said conducting member and assist in confining said suspensions to the intermediate compartment, and an electrode extending into each of the three compartments.

5. Apparatus for use in the process of coating a selected portion of at least one electrically conducting member with a finely divided material from a liquid suspension thereof by electrophoresis, which comprises an electrically insulated cylindrical drum rotatable about its longitudinal axis, two rows of electrically insulated, raised projections integrally formed on said drum in spaced, peripheral relationship and between which electrically conducting members will be supported, said projections having a flattened side and a tapered side with the flattened side thereof being located adjacent the edge of the drum, said rows of projections being in spaced, peripheral relationship normal to the longitudinal axis of the drum with the spacing therebetween defining the portion of the members to be conducted, an electrically insulated trough positioned below said .support for a distance such that a lower segmental section of the drum extends into the trough, a plurality of vertically disposed ribs positioned in said trough and coacting with said rows of projections to define three compartments in said trough with the intermediate compartment being adapted to contain a suspension of the coating material and the two outer compartments being adapted to contain an electrically conducting member which serves to enable electrical contact to be made with said conducting member and assist in confining said suspensions to the intermediate compartment, and an electrode extending into each of the three compartments.

6. Apparatus for use in the process of coating a selected portion of at least one electrically conducting member with a finely divided material from a liquid suspension thereof by electrophoresis, which comprises an electrically insulated cylindrical drum rotatable about its longitudinal axis, two rows of electrically insulated, raised projections integrally formed on said drum in spaced, peripheral relationship and between which electrically conducting members will be supported, said projections having a flattened side and a tapered side with the flattened side thereof being located adjacent the edge of the 7 drum, said rows of projections being in spaced, peripheral relationship normal to the longitudinal axis of the drum with the spacing therebetween defining the portion of the members to be conducted, an electrically insulated trough positioned below said support for a distance such that a lower segmental section of the drum extends into the trough, a plurality of vertically disposed ribs positioned in said trough and coacting with said rows of projections to define three compartments in said trough with each of said ribs being provided with a flattened side and a tapered side with the flattened side thereof being located adjacent the outer compartment of said trough with the intermediate compartment being adapted to contain a suspension of the coating material and the two outer compartments being adapted to contain an 5 electrically conducting member which serves to enable electrical contact to be made with said conducting memher and assist in confining said suspensions to the intermediate compartment, and an electrode extending into 5 each of the three compartments.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain Oct. 16, 1939 

1. APPARATUS FOR USE IN THE PROCESS OF COATING AT LEAST ONE SELECTED PORTION OF AT LEAST ONE ELECTRICALLY CONDUCTING MEMBER WITH AT LEAST ONE FINELY DIVIDED MATERIAL FROM A LIQUID SUSPENSION THEREOF BY ELECTROPHORESIS; WHICH COMPRISES A CYLINDRICAL SUPPORT, A PLURALITY OF ROWS OF RAISED, ELECTRICALLY INSULATED PROJECTIONS SPACED PERIPHERALLY ON SAID SUPPORT FOR SUPPORTING THE CONDUCTING MEMBERS TO BE COATED, SAID ROWS OF PROJECTIONS BEING IN SPACED PARALLEL RELATION NORMAL TO THE LONGITUDINAL AXIS OF THE SUPPORT WITH THE SPACING THEREBETWEEN DEFINING THE PORTION OF THE MEMBERS TO BE COATED, AN ELECTRICALLY INSULATED TROUGH POSITIONED BELOW SAID SUPPORT FOR A DISTANCE SUCH THAT A LOWER SEGMENTAL PORTION OF THE SUPPORT EXTENDS INTO THE TROUGH, A PLURALITY OF VERTICALLY DISPOSED RIBS POSITIONED IN SAID TROUGH AND COATING WITH SAID ROWS OF PROJECTIONS TO DEFINE A PLURALITY OF COMPARTMENTS IN SAID TROUGH WITH ALTERNATE COMPARTMENTS THEREOF BEING ADAPTED TO CONTAIN SUSPENSIONS OF COATING MATERIAL AND THE REMAINING COMPARTMENTS THEREOF BEING ADAPTED TO CONTAIN AN ELECTRICALLY CONDUCING LIQUID WHICH SERVES TO ENABLE ELECTRICAL CONTACT TO BE MADE WITH SAID CONDUCTING MEMBERS AND ASSIST IN CONFINING SAID SUSPENSIONS OF COATING MATERIAL TO THEIR RESPECTIVE COMPARTMENTS, AND AN ELECTRODE EXTENDING INTO EACH COMPARTMENT. 